1. Technical Field of the Invention
The present invention relates to cellular telecommunications networks and, in particular, to a system, method, and apparatus for a cell search within a telecommunications network.
2. Description of Related Art
Reference is now made to FIG. 1 wherein there is illustrated an exemplary cellular telephone network 100. An arbitrary geographic region (hereinafter xe2x80x9cthe service areaxe2x80x9d) 105 is divided into a plurality of contiguous cells 110 schematically represented by hexagons. The cells 110 are then grouped into clusters 115 (outlined in bold to ease recognition). For example, in the frequency plan of FIG. 1, each cluster 115 includes seven cells 110(1)-110(7). It will, of course, be understood that each cluster 115 may have more or fewer cells 110 as required by the selected frequency plan. Each cell 110(1)-110(7) is associated with a base station 120 which provides telephone service to mobile stations 125 using traffic channels. Each cell 110 is also associated with a control channel.
As a subscriber having a mobile station 125 operating within the cellular telephone network 100 moves about the service area 105, the subscriber is likely to leave the coverage area of one cell and enter the coverage area of another cell. If the mobile station 125 is turned on but not at that time engaged in a call, registration of the mobile station""s 125 presence within a new cell 110 is made, as is understood in the art.
When the subscriber is engaged in a call, however, the transfer to the new cell 110 is a more complicated process than a mere registration. In this instance, not only must the mobile station""s 125 presence within the coverage area of the new cell 110 be detected, confirmed and registered, but also the communications link with the mobile station 125 carrying the telephone call must be switched from one base 120 station in the prior cell 110 to another base station 120 in the new cell 110 as the mobile station 125 moves and responsibility for the link is transferred to that new cell 110. The process for performing this action is commonly referred to as xe2x80x9chandoverxe2x80x9d or xe2x80x9chand-offxe2x80x9d. In order to preserve the mobility advantage provided by cellular telephone networks, it is vitally important that the handover of calls in a cellular telephone network occur timely, efficiently, accurately and transparently.
Under the Global System for Mobile Communications (GSM) standard, mobile stations 125 make signal strength measurements within the current cell 110, as well as with respect to neighboring cells 110. The channels measured by the mobile station 125 are identified in a list provided to the mobile station 125 by the cellular network 100. The listed channels include handover measurement channels (usually the control channel) for cells 110 which are potential targets for a handover. The measurements are reported back to the serving base station 120. At the same time, the serving base station 120 measures signal strength of received mobile station 125 transmissions. These mobile 125 and base station 120 measurements are processed by a base station controller (BSC) 130 to determine whether a handover is needed, and also to identify to which cell such handover should be made. Where a handover is needed, the base station controller 130 sends a handover command to the mobile station 125 including information on the channel to be used in the new cell 110 to carry the call. The mobile station 125 then tunes to the channel and continues the conversation. The previously used channel is then released and the BSC 130 transfers the call to the new base station 120 in the new cell 110.
It is axiomatic that technological innovation creates for more advanced telecommunication networks and equipment which deliver more and better services to mobile subscribers than preexisting cellular networks, such as the cellular telephone network 100 shown in FIG. 1. The more advanced networks are often referred to as next generation networks. For example, analog American Mobile Phone Service (AMPS) networks are often referred to as first generation networks, while digital networks, such as Digital-AMPS (D-AMPS) or Global System for Mobile Communications (GSM) are referred to as second generation networks. The emerging Personal Communications Systems (PCS) with Code Division Multiple Access (CDMA), for example, are referred to as third generation networks.
Transition from a preexisting cellular network 100 to a next generation network is often performed in an overlay fashion. In an overlay, the next generation network is gradually deployed alongside the preexisting cellular network. Although the infrastructure of the preexisting cellular network can be replaced wholly by the next generation network, there are, of course, a number of disadvantages associated with this more abrupt approach. For example, mobile stations designed to operate with the preexisting cellular network could be rendered useless, resulting in customer dissatisfaction, and possibly inducing customers to switch to a network maintained by a different operator.
Accordingly, overlaying a next generation network over a preexisting cellular network permits the network operator to gradually phase out the preexisting cellular network. It is noted, however, that next generation networks initially include a limited number of cells in a smaller service area. To promote acceptance of the next generation network, network operators often market dual-mode mobile stations 125, illustrated in FIG. 2. Dual-mode mobile stations 125 allow the subscriber to utilize both the preexisting cellular network as well as the next generation network. The dual-mode mobile station 125 selects a cell in the next generation network when the mobile station 125 is within an area served by the next generation network. However, when the mobile station 125 is in an area that is not served by the next generation network, the dual-mode mobile station registers with the preexisting cellular network 100.
As a subscriber having a mobile station moves from outside the service area of the next generation service area to within the service area, it is desirable for the mobile station 125 to deregister with the preexisting cellular network 100 and register with the next generation network, notwithstanding the fact that the dual-mode mobile station 125 is also within the service area 105 of the preexisting cellular network 100. If the dual-mode mobile station 125 is not engaged in a call at the time, the mobile station 125 deregisters with the preexisting cellular network and registers with the next generation network. However, when the subscriber is engaged in a call, a handover operation must be performed with a cell of the next generation network. Present networks, however, do not provide for inclusion of handover measurement channels for cells in different generation networks. Therefore, a handover from a preexisting cellular network 100 to a next generation network or vice versa is slow, resulting in undesirably long speech interruptions.
Accordingly, it would be advantageous if control channels from different generation networks could be provided to the mobile station.
The present invention is directed to a system, method and apparatus for performing a handover for a mobile station in a mobile telecommunications system including a preexisting cellular network and a next generation cellular network. A cell neighbor list, which includes cells within the preexisting cellular network as well as the next generation network, is transmitted from the serving base station to the mobile station. Responsive thereto, the mobile station measures the signal strength of handover measurement channels associated with the cells in the cell neighbor list, including cells from the next generation network.